SOME ASPECTS OF ESPRESSO EXTRACTION

Jim Schulman

February 2007

ACKNOWLEDGEMENTS

This study on extraction was begun by Andy Schecter. His work has gone in a
different direction, and he is far less inclined than I to suggest large
conclusions on slight evidence; so he has declined being listed as an author.
However, the paper owes its existence, its methods, and much of its logic to
him. We both started thinking about the subject because several roasters and
baristas, including Chris Tacy and Peter Lynaugh were posting on changing how
they dosed when using lighter roasted Single Origin coffees. My thanks to Luca
Costanzo for telling me about the "French curve" dosing swipers coming into use
in Australia and Scandinavia. The move towards SOs in the world wide barista
community is leading to a much wider discussion of dosing practices, and I
apologize for not citing everyone involved. My thanks to James Hoffman and
Alistair Durie, who issued a call for papers via Coffeed.com. Finally, a huge
acknowledgment to Ted Lingle, whose classification of coffee flavors by their
molecular weight may have provided the best key for tuning the flavor of an
espresso shot

ABSTRACT

The paper presents and discusses data from three related aspects of espresso
extraction:

INTRODUCTION

Some aromas that promise joy from a brewed cup of coffee elicit dread when
smelled in a shot of espresso. Why do so many great brewed coffees taste awful as
espresso?

The espresso community has long established rules about this. Roughly stated,
coffee for espresso has to be low in acidity, not too lightly or darkly roasted,
heavy bodied, and contain a significant percentage of dry processed beans. But
despite everyone's experience with spectacularly acidic or bitter shots, despite
that Pavlovian anticipatory cringe every espresso hound has developed, there is
no convincing logical reason for this.

Espresso is more concentrated than regular coffee; so strongly flavored
coffees are said to become too strong as espresso shots. But the oils and crema
of espresso buffer its extra strength. Moreover, the bad taste of these coffees
done as espresso seems less about it being too strong, and more about being
unbalanced. The explanation for the unsuitability for espresso of some great
brewing coffees may not be in espresso's strength, its extraction of lipids, or
its creation of crema. Rather, it may be that espresso brewing extracts flavors
differently, and alters their balance in the little cup. This paper tries to get
some answers to what this difference is, how it occurs, and how it can be
controlled.

ESPRESSO TASTE AT DIFFERENT LEVELS OF SOLUBLES YIELD

summary of results Under-extracted espresso
tastes excessively sharp and acidic. Properly extracted espresso has the
sweetness to balance the acids and bitters. Over-extracted espresso tastes dull
and tarry, or just bitter-sweet.

coffee strength, solubles yield, and taste
Everyone knows espresso is stronger than regular coffee; the same tablespoon of
coffee brewed into six ounces of regular coffee is used to make one ounce of
espresso. But this is not the whole story. Not all the ground coffee goes into
the cup or shot. In properly brewed coffee, about 18% to 22% of the ground
coffee is dissolved into the water, while the rest is spent grounds; the range
in espresso shots is wider, running from 15% to 25%. This proportion of ground
coffee that ends up in the cup is called the solubles yield; a
brew with too low solubles yield is called under-extracted, while
one with too high solubles yield is called over-extracted.

It is important to understand that solubles yield and brew strength are two
separate things. For instance, imagine that the six ounce cup of brewed coffee
was extracted at 24%. Its strength would be 24% of the coffee measure divided by
6 ounces or roughly 4% per ounce. If the same measure in a shot of espresso was
extracted at 16%, all of it would still be in one ounce of water, so the
under-extracted espresso would still have four times the strength of the
over-extracted brewed cup(1).

If we imagine tasting the same amount of extraction in less and less water,
we would notice the same aromas and tastes getting stronger and stronger. In an
analogy, it's the volume getting louder, but the music staying the same. How
about if we taste more and more extracted ground coffee in the same strength of
brew? Roasted coffee has over a thousand different flavor chemicals. Some of
these dissolve quickly, some more slowly. As the grounds become more extracted,
the slower dissolving flavors become more prominent in the brew. So, in our
analogy, it would be the volume staying the same, but the music gradually
changing.

This paper is about the solubles yield and how it a affects the taste; it is
not about brew strength. The brew strength of a standard espresso shot has about
120 parts per thousand coffee solids. The shots I made for this paper ranged
from about 100 to 160 parts per one thousand, with weaker and stronger shots
occurring at all solubles yield levels. In the data analysis, the effect, if
any, of the brew strength, is removed from the results.

how to taste for proper extraction With a
thousand flavor compounds to play with, no two coffees are, or ever will be, the
same. So how can there be any way of knowing what extraction is best for what
coffee? Fortunately, these flavors can be grouped into a few large families, so
that all the members of a given flavor family extract in similar ways. This work
was done by Ted Lingle, who grouped the flavors by molecular weight, with the
light weight ones dissolving quickly, and the heavy weight ones dissolving
slowly:

There are two light weight, fast dissolving, families that are fully present
even in under-extracted coffee.

Fruit acids have fruity or floral aromas and flavors, crisp
tastes in sweeter brews, and sour tastes in less sweet ones. These dissolve
the fastest

Maillard compounds have the aromas and flavors of toasted
grain, wood, tannins, or nuts; and tastes which are sharply bitter in less
sweet brews, and warm, round, and malty in sweeter ones. These dissolve more
slowly than the fruit acids, but will still all get into even the most
under-extracted cup or shot

There are also two heavy weight, slow dissolving families which require high
solubles yields to reach their full strength.

Caramels have caramel, vanilla or chocolate flavors and a
sweet taste. Since almost all sugars in green coffee are caramelized during
the roast, these are the primary source of sweetness in coffee. Dark caramels,
which taste bitter-sweet, dissolve more slowly than light caramels, which
taste more sugary. Some light caramels will get into even lower extractions,
but require higher extraction rates to be completely dissolved.

Dry distillates are reduced (burnt) caramels and maillard
compounds that become dominant in dark roasts. They have the aromas and
flavors of clove, tobacco, peat, or turpeny, a dully bitter, ashen taste in
less sweet brews, and a bitter-sweet molasses taste in sweeter brews. These
dissolve very slowly, but are tasteable at very low concentrations. Their
presence is usually a good reason to keep extraction levels fairly low.

This flavor classification(2)
provides a road map to a balanced coffee extraction, either for brewing or in
espresso. While describing the taste of coffee both accurately and in detail is
an art; it is fairly easy to sort the tastes and smells into these four broad
groups. It is even easier to tell if the coffee is under-extracted, properly
extracted or over-extracted:

an espresso taste
test Does this systematic approach to proper extraction levels work for
espresso? I tested this idea with an acidic blend roasted so lightly as to be at
the limit of current US espresso making practice. I wanted to see if
manipulating the extraction could get comfortable, rather than bleeding edge,
shots from this blend. I also tested the same extraction levels on the blend
roasted to the more usual Vienna Roast level.

I had previously worked out a way to measure and control the solubles yield
in espresso by varying the dose and grind (see next section). I was aiming for
extractions from 16% to 24% solubles yield. To get this, I used an LM triple
basket, and did shots at 12 grams, 14.5 grams, 17 grams, and 19.5 grams,
grinding each dose so get normal double shots in around 25 to 35 seconds. I did
six shots at each dose level, and measured the extraction level.

Four of the shots at each dose were done with the City (Northern Italian)
roast, two at a Vienna (Southern Italian) roast. The coffee was a 50/50 mix of
Idido, a DP Yrgacheffe with floral, citrus, chocolate, and green tea flavors,
and Cenaproc, a WP Bolivian Bourbon with an apple acidity and very sweet
marzipan caramels. I rated the aroma, mouthfeel, and crema appearance of each
shot on a zero to ten scale. I also rated the balance of acidic to bitter
flavors, with zero as most acidic, ten most bitter, and five as neutral.
Finally, I rated the overall sweetness from zero to ten.

There were two flaws to this test; so these results need to be confirmed.

When I came up with the acid/bitter scale, I did not appreciate the nature
of light maillard flavors. These are both bitter and fast extracting. I was
under the mistaken impression that all bitter flavors extract slowly. This
flaw is ameliorated since the blend in this test was mainly fruity, with few
light maillard flavors. So under-extraction is signaled by a excessive
sourness, not bitterness.

The tasting was not blind; I knew what the predicted yield was when I
tasted. Since I did not know the actual yield, it is possible to detect and
compensate for any bias. My tasting scores followed actual yields more closely
than the predicted ones, so I do not believe bias was a major factor.

The crema, mouthfeel and aroma had no relation to the solubles yield(3).
It is possible to get excellent and alas lousy shots in these aspects at all
yields

As the extraction model predicts, higher yield shots tended to be sweeter
than lower yield ones for all roasts(4);
but lighter roasts responded to yield differences more dramatically than dark
ones. This is logical, since dark caramels are not as sweet as light ones. At
very high yields, the light roast shots became distinctly caramel flavored,
indicating over-extraction. Extractions around 23% for the light roast, and
around 20% for the dark one tasted the best balanced

Higher yield shots tended to a more bitter balance, and lower yield ones to a
more acidic balance(5).
This was equally true for light and dark roasts, although the darker roast was
of course more bitter over the entire series. This effect would not have
occurred with a light roasted, low acid coffee dominated by light maillard
flavors, such as a Monsooned Malabar. In such a coffee, the taste would have
been bitter at all yields, with increasing sweetness at higher yields

While this taste test is hardly conclusive, it does support the relationship
between solubles yield and taste implicit in Ted Lingle's work on the flavor
chemistry of coffee. It should encourage people who enjoy espresso to experiment
with varying the solubles yield in their shots, and to confirm that the shot's
taste can be tuned by doing this.

So how does one control the solubles yield?

SOLUBLES YIELD BY DOSE AND GRIND

summary of results For a given basket, grinder,
and machine, higher doses lead to lower extractions, lower doses to higher
extractions.

measuring solubles yield As usual, Andy Schecter
had asked the right question and done the right thing. He asked "How do you know
a ristretto is really stronger than a normale? Maybe the ristretto has less of
the puck in it." Then he started weighing shots and pucks.

I was obsessing about the extraction process when he emailed me about his
project, and I realized he had figured out a way to get control over the
solubles yield in extraction. The yield is the percentage reduction in puck
weight as its constituents are dissolved into the espresso. If one weighs the
puck before and after the shot, one can figure it out. If one does it over and
over again, for different baskets, doses, coffees, shot times, shot weights, and
puts the data through the statistical meat-grinder; one can detect the patterns
and learn to control the extraction. I was off and running.

The puck weighing has some additional requirements. The puck is wet after the
shot, and needs to be oven dried before weighing. Fresh ground coffee holds some
water, so a grind sample also needs to be oven dried and the initial puck
weights have to be adjusted accordingly. Finally, some grounds from the puck
cling to the shower screen and group gasket after the shot; these need to be
recovered with careful brushing.

There are two sources of error in the yield results. Any ground coffee not
recovered for the weighing would reduce the puck's weight and register as
increased yield. The puck's moisture contains extracted coffee which is added to
back to the puck's weight while baking, thus registering as decreased yield.
Marino Petracco, in the extraction chapter in Illy, cites 25% yields for Italian
dosing standards. My measurements were about 2% to 3% less and may be an
artifact of this soggy puck error. However, since these sources of error are the
same for every observation, the relationship between yield and its predictors is
unaffected.

controlling solubles yield My observations ranged
from 15% to 25% of puck weight extracted. There were two outliers, one at 12%
and one at 26%, both coming from extractions well outside normal espresso time
and volume limits. The next task was to find out which of the controllable shot
parameters can be used to set the yield.

Grind setting was not consistently related to yield except when using the
same basket, coffee, and roast level. Different coffees, roasts and baskets
require different grind ranges.

The major predictor of yield is the weight of the puck divided by the hole
area at the bottom of the basket (P/A). I measured the diameter of the
hole-punched area at the bottom of the basket and squared it. This figure is not
the actual hole area; but is directly proportional to it and easy to determine.
The P/A measure can be simply understood as the depth of the puck in the basket;
a fat or deep puck extracts less solubles than a thin or shallow puck. When the
puck is conical rather than cylindrical, so that the top surface is larger than
the surface against the filter holes; the P/A measure can be understood as the
average length the water has to travel through the ground coffee. It is
operationally identical to the depth of the puck in a perfectly cylindrical
basket.

This straight line relation explained about 75% of the variations in yield,
and the predictions had a standard error of 1.8% yield level. Moreover, the
formula using P/A remained the same for all baskets, coffees, and roast levels
for the same machine. In the graphic below, which applies to the Elektra
Semiautomatica group, an LM double basket dosed at the Italian standard of 12 to
14 grams would yield about 22% to 20%, and only 17% to 18% when dosed at the US
standard of 17 to 19 grams.

Andy's observations with his highly modified Silvia also showed the P/A
relation with the same predictive power. However, the slope of his observations
is different from mine. This graph shows only the readings I took on the
Elektra

Changes in shot time and weight also affected the yield, but the effect, when
shots were cut as they blonded, was surprisingly small(6).
Adding these variables to the predictor equation only improves the model's
overall accuracy by about 3%, and only reduces the standard error in the yield
prediction from 1.71% to 1.65%. Given the added complexity of the formula, it
has no everyday use.

The first reaction I got to this work is that low dose shots probably taste
weak when compared to high dose shots. This is true when a poorly trained
barista pulls a shot with roughly the same volume for both doses. If one attends
to the color of the stream when stopping the shot, the solids concentration in
the cup remains fairly constant regardless of dose. In this experiment, in the
24 shots I made for the taste test, there was no relation between dose and the
amount of solubles in the cup. In the graphic below, shot concentration is
measured as the weight reduction of the puck (solids getting into the cup)
divided by the weight of the shot. The unit is parts per thousand, and the
dashed line shows the 120 parts per thousand standard.

In practice, on any given machine, one can use a simple straight line P/A
formula to calculate the yield for any basket and coffee. However, since it is
different from machine to machine, and maybe grinder to grinder, all baristas
will have to work it out for their own setups. Fortunately, it is not required
to know the exact figures. One only needs to know that going to a lower dose and
finer grind in the same basket increases the extraction. One can taste the
shots, analyze their flavors using the taste model in the last section, and make
dosing changes in the right direction until one has found the correct dose for
every coffee(7).

Why does it work this way? Why does only the dose and basket shape matter a
lot, while the shot time and volume don't matter much? To answer these
questions, one needs to learn more about how the coffee extracts during the
course of an espresso shot.

PUCK EXTRACTION DURING THE SHOT

summary of results The yield does not depend a
lot on shot time or weight, because the puck is extracted almost as far as it
will go within the first 20 seconds of the shot. I'm not sure why the total
amount extracted is nearly done by twenty seconds, or why it varies by dose.
That doesn't stop me from speculating about it at the end of this section

the taste of the early, middle and late part of an
espresso shot There's an old alt.coffee exercise: brew an espresso shot
into three cups; the first 10 seconds into the first cup, the next 10 seconds
into the second cup, the rest into the third cup. Then taste(8).

The first cup will taste extremely sharp, sour, and strong.

The second cup will taste sweet and creamy, but bland.

The third cup will taste watery and slightly bitter.

When we discussed this, we assumed that the puck was extracting evenly
throughout the shot; so that the first cup represented the 0% to 7% solubles
fraction, the next cup the 8% to 14% solubles fraction, and the third cup the
15% to 21% fraction. However, the last section shows that the degree of
extraction is relatively immune to shot time and weight. This leads to a, in
hindsight, much simpler interpretation of the three cups data. The first cup
represents the early extraction, the second cup the late extraction, and the
third cup a diluting of the shot. If the puck loses almost all its solubles into
the first 20 seconds and 50% weight of the shot; then the near immunity of the
yield data to shot timing is explained. All the shots in the extraction data set
were within normal espresso parameters. So the only thing that varied is by how
much the full extraction, present in the cup after the first 20 seconds, was
diluted by the rest of the shot.

Is there any way to confirm this supposition?

intra-shot solubles yields The obvious solution
is to check the solubles yield of the puck at various stages over the course of
a single shot. This can't be done with just one shot, but it can be done with
six shots stopped short at staggered intervals which use the same coffee, dose
and grind. I did two of these sets, stopping the first shot in each set at the
first drop, and the subsequent ones at 6 second intervals after that, out to 30
seconds (actually 35, since the dwell time is around 5 seconds). I used a high
dose, because I was also disecting the puck (see below), and the final yield is,
as predicted, on the low side. The graph shows the averaged readings of the two
sets(9).

This confirms that the bulk of the solids extraction takes place in the first
2/3rd of the shot, and begins to explain why shot time and weight play such a
minor role. What it doesn't explain is:

Why does the extraction stop after 20 seconds, even in the case of a high
dose, low yield shot?

Why do low dosed shots extract more than high dosed shots?

getting inside the puck while it is brewing I
haven't gotten very far with these questions; and I may be looking in the wrong
direction. But, I think the answer lies in what happens inside the puck during
the shot. This is pretty much a "black box," but with some theory and some
tricks, one can begin to crack into it.

The theory is based on the idea of percolation. This has
nothing to do with those accursed coffee percolators from the 50s. Rather, it
looks at the brewing process as water flowing through a column of ground coffee.
Since the puck is, sort of, a column of ground coffee, and since the solubles
yield depends on how high this "column" is, it seems an appropriate way to think
about the problem.

Discussions of percolation(10)
can get confusing very fast. The ground stuff is "coffee," the liquid coming out
of the bottom is "coffee," the stuff going from the grinds to the liquid is
"coffee." One needs to come up with a new vocabulary to prevent total confusion.
Here goes:

Grinds: The ground coffee.

Liquid: The water going through the grinds and becoming
coffee.

Solubles: The stuff going from the grinds into the water,
turning it into coffee.

The mental model percolation is real simple. The liquid goes into the top,
picks up solubles from the grinds at the top, and becomes saturated. Once it's
saturated, it can't pick up any more solubles, so the solubles stay inside the
grinds at the bottom until the top grinds are exhausted, and the liquid reaching
the bottom is less saturated. In other words, the percolation column brews from
the top down.

The problem is that this mental model doesn't fit our facts. In this model,
it doesn't matter how high the column is, or how coarse the grinds. It brews
from the top down. Send through enough water, and it will extract as far as you
desire. But the intra-shot graph shows that the extraction basically levels off
after 20 seconds. The dose by extraction data shows that the amount of
extraction in normal shots varies almost entirely by puck height, and very
little by time or volume.

Time to gather some data and make the model more complicated. The data is a
series of shots stopped at the 6 second intervals; with each puck divided into
three horizontal slices; and each slice's brew strength measured. The more brew
strength, the less coffee has been extracted. This provides a picture of how the
top, middle and bottom of the puck extract over the course of a shot.

The puck sections were oven dried, and brewed at exactly 4 grams powder to 80
grams of water. These brews were compared to the fresh coffee, also oven dried,
brewed at 4 grams (100%), 2 grams (50%), and 1 gram (25%) per 80 grams water.
The comparison was by TDS meter. I did two series of measures.

The three pairs of horizontal lines show the TDS of the fresh coffee at 100%,
50% and 25% brew strength. They provide a rough measure of the extraction levels
of the puck sections, with the 25% line indicating a full extraction. The top of
the puck is shown by the blue lines, the middle by the green lines, the bottom
by the red lines.

The measure at time zero is at the first drop. It shows the puck state when
it gets completely soaked. And it leads to out first real world revision of the
simple percolation model: grinds absorb liquid. The liquid absorbed at the top
is water, while the liquid at the bottom has all the solubles it picked up at
the top. So, when the percolation column becomes completely soaked, but before
any liquid comes out, solubles have been transferred from the grinds at the top
to the grinds below. Below a certain depth, the grinds actually are charged with
solubles, rather than losing them (the same trick is used to recharge decaf
coffee after the caffeine has been removed)

The next revision comes from looking at the shape of the lines. They are
roughly straight, rather than the asymptotic curves one expects from the simple
model (the intra-shot extraction graph in the previous section is an asymptotic
curve, it reaches a limit). The water is not marching down the puck column in a
straight line. The puck has become soaked, and has turned into a slurry. The
fresh water enters this slurry and mixes with it. The mixed water is pushed out
of the bottom. The proper mental model is of a big pail filled with liquid and
grinds with a filtered hole in the bottom where liquid but not grinds flows out.
The water in the pail is both brewing and being diluted by the in flowing water.
It's the mixed up result that emerges from the bottom.

Do these two revisions to the percolation model explain the extraction
results?

A bit; but not like a slam dunk. In the soaking phase, the thinner the puck,
the less overloaded with solubles the bottom layer becomes. Since the bottom
layers can't extract their own solubles until they've got rid of the excess ones
from the top, thinner pucks help along the extraction. In the progressive
dilution phase, the bottom of a thinner puck will get more fresh water and
extract further. These two factors together may explain the results.

There's a lot more to the story that I don't know or can't document. Thinner
pucks require finer grinds, so the extraction is probably accelerated by that.
The fines, and finer particles in general, migrate toward the bottom of the
puck, so when the extraction at the bottom gets going, it probably proceeds
faster than at the top. Top down extraction, soaking and dilution phases,
thinner and thicker pucks, finer and coarser grinds: there's a lot happening
during an espresso shot. These intra-shot measures are a first peek, but there's
a lot more looking required.

CONCLUSION

None of these complications should be allowed to obscure the basic point: the
taste of espresso varies by the level of extraction, and that level can be
controlled.

And this result points to the irony of unintended consequences.

In Italy, espresso is a mass consumption item, mostly made from coffees of
the same low quality as is found in supermarkets everywhere. Since the aromas of
such coffees are not all that great, staling is of little consequence, while
keeping doses precise and yields high is of great consequence, since one needs
to extract every iota of caramel to make the shots palatable. So the ground
coffee sits in dosers going stale, but is precisely dosed, 6.5 grams into single
baskets, 13 into doubles.

In the non-Mediterranean world, espresso is specialty coffee(11).
Cafe owners rightly noticed that the ground coffee was going stale in the
dosers, and went to alternative dosing methods. What they didn't notice is that
dosing by leveling the freshly ground coffee to the basket's rim, a dose far
higher than the Italian norm, gets solubles yields of 16% to 20%, rather than
the 20% to 24% one gets with a properly adjusted doser. "Specialty Espresso" was
almost always under-extracted

At these low extraction levels, high grade coffees become a bane rather than
a boon, producing jarringly acidic or sharp shots. So, in the specialty coffee
world, there is a feverish search on for ultra-sugary high grown coffees that
are still sweet when roasted light and under-extracted. When such coffees are
not available, one gets the ubiquitous medium-dark roasted blends that are a far
cry from the quality of the specialty coffees sold for regular brewing.

And all this because of an unintended consequence of using fresh coffee. I
think it's high time for baristas to relearn their dosing.

As a final note: since posting early drafts of this paper, I have found out
that this is changing already. In Scandinavia and Australia, many top competing
baristas are replacing their fingers with curved swipers that scoop out ground
coffee below the rim level of the basket. By having a french-curve like set of
these, they can efficiently vary the dose in a workplace or competition context.
I'm sure these "3rd wave" dosing tools will become much more prevalent and
developed as word on working the solubles yields gets out.

NOTES

(1) For a more detailed explanation, consult Ted
Lingle, The Basics of Brewing Coffee, 1996, SCAA.

(2) The classification used here differs from the
one in Ted Lingle, The Coffee Cupper's Handbook, 2001, SCAA. He
divides flavors into three main groups, enzymatic, sugars browning, and dry
distillates. However, the herby and nutty sub groups contain flavors that derive
mostly from Maillard reactions between sugars and amino acids. These are the
flavors that typify toasted grains, malts, smoked meats and barbecued foods. I
believe they deserve their own overall classification for three reasons: they
are associated with sharp or bright-bitter tastes, rather than the sour or sweet
tastes of the neighboring groups; they have similarly fast solution rates; and
they are mostly produced in the part of the roast running from 300F to the first
crack, and hence can be roast-profiled as a group.

(3) Aroma slightly improved at high extractions,
but the effect is only marginally significant (0.95):

Mouthfeel and crema are, as expected, well related to the concentration of
the shot. Here are the relevant partial regressions:

(4) The residual correlation of sweetness to yield,
after removing all other factors, was 56%, with a t-value of 3.12.

(5) The residual correlation of acid/bitter to
yield, after removing all other factors, was 73% with a t-value of 4.82.

(6) The t-value for the simple linear regression of
filter.area/dose to yield is 7.89; if one uses the best predictor:
filter.area/dose*log(shot.time*shot.weight) the t-value rises to 8.69. The graph
is shown with the predictor inverted to the more sensible dose/filter.area, so
the regression line is transformed to a hyperbolic curve. With t-values this
huge, the magnitude of the change in the correlation coefficients from 75.2% to
78.3% has a high degree of certainty, and shows that while shot time and weight
do expalin some of the yield changes, they definitely don't explain a great
deal.

The small effect of shot time and weight/volume is probably an artefact of
cutting shots when the flow blondes. This cuts fast flowing shots short, and
lengthens the slow flowing ones. This practice counteracts the physically
mandated higher extraction rates of fast flowing shots. However, since cutting a
shot as it blondes is proper barista technique, the result applies to actual
shot making.

(7) Dosing changes can also affect the shot because
higher doses can come into contact with the shower screen, while lower doses do
not. This depends also on the height of the basket. M. Petracco warns against
puck contact with the shower screen in chapter 7 of Illy and Vianni, eds,
Espresso Coffee: the Science of Quality, 2005, Elsevier.

...the host inclines to overdose to serve the guest the best
possible cup. This practice is risky ... because an excessive amount of ground
coffee does not permit sufficient expansion during cake wetting.

On my Elektra, the shower screen is like a third rail, so all the data in
this paper are from shots with head space. In other groups, shots seem to
survive compression by the shower screen without harm.

(8) I could not find the original alt.coffee posts,
this early
post is pretty representative.

(9) The 12 second reading only contains one
observation, since I messed up the other one.

(10) Chapter 7 in Illy, as cited in note 7 is a
discussion of espresso percolation. It focuses mainly on how the puck
dynamically affects the flow, and helped me appreciate the important role of the
grinds absorbing liquid. However, it does not go into the timing of solids
extraction. Discussions of percolation for instant coffee, pp 127-128, Clarke
and Vitzthum eds, Coffee: Recent Developments, 2001, Blackwell
Science, are frightening; but they emphasize the role of how the initial
percolation column gets wet (top down versus before starting the percolation),
and how hard it is to get a fully extracted or in the instant coffee case an
insanely over-extracted, output.

(11) Historian Jonathan Morris in his
inaugural lecture given at the University of Hertfordshire, November, 2005, The Cappucino
Conquests, tells the story from a British perspective.